1. Field of the Invention
This invention relates to controlling the temperature of a multiple-segment fiber glass bushing and, more particularly, to actively monitoring and controlling the temperature of each individual bushing segment.
2A. Technical Considerations
Glass fibers are produced by drawing multiple streams of molten glass at a given rate of speed through orifices or nozzles located in a heated container known in the fiber glass industry as a bushing. The bushings containing the molten glass are electrically heated and maintained at given temperatures to provide molten glass at the orifices or nozzles at a desired viscosity. The fibers drawn from the orifices or nozzles are gathered after they solidify into one or more strands and are collected on a collet into one or more forming packages.
In recent years, bushings have increased in size so that bushings having 800 to 2,000 or more orifices or nozzles are commonplace in the industry. It is also common practice to produce more than one strand from these larger bushings by winding, for example, four strands from a single bushing. Such an arrangement is generally referred to as a split-bushing. Typically, this is accomplished by dividing the bushing into sections with each section providing one strand. Splitting the bushing in this manner to produce more than one strand requires precise control of the bushing section temperatures so that the strands produced and collected on the collet have the same yardage, i.e., the same yards per pound of glass or, viewed in another way, the same weight of glass strand per package collected on the collet for a given period of time.
The development of technology for adjusting bushing heat patterns and controlling formation of the individual strands, and in particular the coefficient of variation in the filament diameters, has progressed from moving manual fin coolers, which provide large but somewhat imprecise bushing adjustments, to three and four terminal controllers which adjust the electrical current in each section of the bushing by shunting a controlled current around sections of the bushing to produce variable heating. However, with these types of controllers, there is no feedback of external process conditions such as overall bushing temperature. Thus, a long term glass flow adjustment could be performed but short term temperature variances persist.
It would be advantageous to provide a segmented bushing temperature controller that actively monitors and controls the temperature of each bushing segment by monitoring the status of the thermocouples as well as the overall thermal condition of the bushing.
2B. Patents of Interest
U.S. Pat. No. 3,246,124 to Trethewey teaches a temperature control device for a glass fiber forming bushing wherein the power to the bushing is based on the average temperature of the bushing.
U.S. Pat. No. 3,540,001 to Gormley et al. teaches a control circuit having at least two controllers which are operatively connected to provide interrelated operation. For example, a first controller can operate on a first process variable to provide a remote set point for a second controller. The second controller, in turn, operates to control a second process variable which directly affects the first variable.
U.S. Pat. No. 4,024,336 to Jensen teaches a circuit for controlling the temperature of a two-segment fiber glass forming bushing. The bushing control uses two temperature controllers and two full wave variable impedance devices to regulate the current transmitted from a power transformer to the two segments of the bushing.
U.S. Pat. No. 4,149,022 to Hrycik teaches a power control system for electrically melting glass in a vertically oriented type glass melting furnace. Multiple sets of electrodes are positioned at various levels within the furnace and a control means directs power to the electrodes, as necessary, to distribute electrical energy in the furnace.
U.S. Pat. No. 4,162,379 to Sebens et al. teaches an apparatus for maintaining a thermal system at a stabilized condition by reducing the control error. A feedback signal from the system is derived by linearly combining functions of the current through an electrical heating element and the voltage thereacross, respectively.
U.S. Pat. No. 4,515,614 to Barkhau et al. teaches a method of controlling molten glass temperature in an electrically heated forehearth. Separate circuit controls, temperature sensors and temperature set point control means are provided for electrodes on the side walls of the furnace. Glass temperature across the flow path is controlled and adjusted by separately controlling the flow of current on each side of the side wall.
U.S. Pat. No. 4,546,485 to Griffiths et al. teaches a method of equalizing the temperature of a fiber glass forming bushing by averaging the temperature across the bushing face plate and controlling the power input to the bushing in response to the measured average. The two halves of the bushing are controlled by placing a variable resistor in the power supply lines to the bushing and adjusting current input to the side in response to variations in weight or yardage measured for the two strands produced by the bushing.
U.S. Pat. No. 4,594,087 to Kuhn teaches a three terminal controller for a fiber glass bushing. The bushing temperature is measured by a plurality of thermocouples which produce an average temperature reading that is used to control the power fed to the bushing. The three terminal controller is connected across two sections of the bushing to regulate current flow in the two sections in order to control the amount of fiber produced in each section.
U.S. Pat. No. 4,657,572 to Desai et al. teaches a bushing balance controller which measures voltage drop across each segment of a multiple segment glass fiber forming bushing and the current flow in the bushing. These measurements produce error signals proportionate to the difference in the set point temperature and the instantaneous temperature of each segment of the bushing. Current is diverted from each segment of the bushing whose error signal is greater than the average error signal and current is supplied to the overall bushing when the sum of the error signals is greater than a preset value.
U.S. Pat. No. 4,738,700 to Grundy teaches the positioning of thermocouples in a fiber glass forming bushing at locations that eliminate or reduce signal noise. A controller averages the monitored temperatures of the thermocouples which, in turn, is used to regulate the current passing through the bushing.
U.S. Pat. No. 4,780,120 to Varrasso et al. teaches a balanced bushing controller for a multiple section glass fiber producing bushing. Thermocouples connected to all but one of the bushing sections inject power to those sections. A thermocouple connected to the last bushing segment controls the application of electrical energy to the entire bushing.
U.S. Pat. No. 4,787,926 to Varrasso teaches a digitally controlled apparatus for varying the temperature of a glass fiber forming bushing over a period of time. The digitally controlled apparatus is used in combination with a constant speed winder so as to adjust the temperature of the bushing to maintain a constant glass fiber diameter as the fiber is wound on the winder.